why 4-20mA? Why not from 0?

B

Bob Peterson

The zero elevation also gives you the ability to use the voltage drop this has to create to power the transmitter, thus allwoing for 2 wire transmitters.
 
A

Arnold Dillon

The signals could just as easily start at 0 milliamps. One of the main reasons for choosing a non-zero value was that it gives a means of error checking called a "Live Zero." If the loop reads 4 milliamps then a zero measured value is
detected, but a valid current loop exists. If the same loop reads 0 milliamps then an open loops exists, and something is wrong.
 
There is no technical reason why 0 ma cannot be used IF you are using a field powered device (usually called 4 wire transmitters). If however
you are using loop powered devices (usually called 2 wire transmitters), the device itself needs to obtain power to operate, even with a zero indication. The power available in that 4 ma or 1 V offset is sufficient to provide this function, and still retain the open loop detection as well.

John Beck
Control System Specialist, Retired
 
W
The reason is that if there is a line break, the current drops to zero in a 4-20 mADC system, while in a 0-20 mADC system this alarm feature does not exist.

Walt Boyes

---------SPITZER AND BOYES, LLC-------------
"Consulting from the engineer
to the distribution channel"
www.spitzerandboyes.com
[email protected]
21118 SE 278th Place
Maple Valley, WA 98038
253-709-5046 cell 425-432-8262 home office
fax:801-749-7142
--------------------------------------------
 
J
Apart from the wire break detection, another reason for 4-20 mA is that two-wire devices that power themselves from the 4-20 mA need the guaranteed 4 mA to operate. I.e. the internal workings of this type of device is that about 4 mA is used to power the circuitry, and about 0-16 mA is shunted past, thus adding up to the 4-20 mA signal.

==================
[email protected]
www.smar.com
 
hello guys,

can you please give me an unique reason for why we are using 4 to 20mA instead of 0 to 20mA

jyothi
 
I can't believe how many of you think the 4 mA is just for break detection. If that was the reason why not 1 mA?

It's to power the transmitter.

Some of the old guys will remember when it was 10 -50 mA, as electronics improved we no longer needed 10 mA to power the electronics.
 
Top